1. Field of the Invention
The present invention relates to a process for the fractional desublimation of pyromellitic acid dianhydride.
2. Description of the Prior Art
The economical preparation of very pure, sublimable, organic substances, i.e., such substances which, under working conditions, change directly from the vaporous to the solid state, from a hot gas mixture which is mainly taken from a reactor for the preparation of said substances and which, as an impurity, contains by-products which, under the required working conditions, remain in the waste gas in the form of vapor, has gained significance during the past few years.
The separation of sublimable substances from gas mixtures containing them, is known. All these known processes have the joint requirement that the hot gas be cooled on cooled surfaces to such an extent that the desired degree of separation is reached for the substance to be obtained. Here, the heat exchanger surfaces usually become coated with the product so that a periodic removal is necessary. Large-volume sublimation chambers can be emptied easily by means of scrapers and brushes. However, because of the relatively small chamber surfaces, the throughput or the space-time yield is small. The presently most economical separators are ribbed pipe condensers which, however, cannot be emptied mechanically. In this case, the sublimate or the condensate is obtained by a periodic heating of the ribbed pipes to temperatures above the solidification point of the obtained products and by the drainage of the products into collectors. The disadvantage of this process is the requirement that two condensers have to be operated alternately, one of which is loaded, while the other one is melted off. In addition, such a condenser is useless for products which have a very high melting point and/or decompose at their melting point. Among the products that cannot be obtained in ribbed pipe condensers of conventional construction is, among other products, pyromellitic acid dianhydride (PMDA). First, the melting point at 285.degree. C. is so high that a melting off would present technical difficulties, and secondly, the product will discolor when the PMDA is heated to melting temperature. This discoloration cannot be eliminated by means of the methods known up to date, unless distillation is carried out which, because of the high melting point of the product, also presents considerable technical problems.
Several known processes are indicated in literature concerning the separation of PMDA from reaction gases:
Thus the use of sublimation chambers was suggested several times, in which case, in order to improve effectiveness, cooling surfaces and baffle plates may be installed, or a cooling of the gases is carried out by injecting water, in which case the cooling must, however not exceed 555.degree. C./sec. The disadvantage of the latter process is the increase of the thaw point of the water, which results in an increased formation of pyromellitic acid (PMS). The mixing of the hot reaction gases with cold air seems to have a more favorable effect. However, this method results in fine crystals which, because of the increase in flow speed, are increasingly discharged from the sublimation chambers. In order to avoid such losses, filter or water washes have been suggested.
The water wash of all reaction gases is much simpler. In this case, the whole product accumulates as PMS which then, according to various methods must again be dehydrated into dianhydride.
According to another process, the hot reaction gases which contain PMDA, are brought in contact with pneumatically transported solid balls which absorb the heat and thus coat themselves with the material to be sublimated. In a separate facility, the solid balls are cleaned (baffle effect) and cooled mechanically before they are transported back into the sublimation chamber. Understandably, this process requires complicated equipment.
It is also known that the PMDA accumulating as a result of the gas phase oxidation of tetra-alkylated benzene (for example, durene) is rendered impure through a number of by-products, the vapor pressures of which are higher than that of the PMDA and/or, because of their more extensive dilution in the reaction gas, have a lower thaw point. Among these are: Trimellitic acid anhydride, dimethyl-, monomethyl-phthalic acid anhydride and phthalic acid anhydride as colorless impurity in addition to a number of other dark-colored compounds. In order to remove these impurities, refining is required. The suggested processes usually deal with a treatment of the crude product with solvents, especially ketones or solvent mixtures, while another process became known in which hot inert gas or air, at temperatures between 100.degree. and 200.degree. C., at a quantity of 1 to 150 kg gas/kg crude PMDA is directed over or through the crude product, until a degree of purity of 99% is reached.
It is also known that pure PMDA can be obtained when the desublimation of the crude product is carried out at temperatures between 130.degree. and 200.degree. C.
In a newer process, PMDA-containing reaction gases, which were precooled to a temperature which is 5.degree.-10.degree. C. above the thaw point of the PMDA, are directed vertically through a thermostatized, perforated cooling surface, which makes it possible to cool the gases to a temperature above the thaw point of the by-products, in which case, pure PMDA accumulates on the flow-in side of the cooling surface. This pure PMDA can be easily separated mechanically. However, apparatuses of this type can only be set up for a large-scale PMDA preparation with the expenditure of considerable costs.